This invention relates to the decolorization and stabilization of alkylphenyl esters of phosphoric acid. The production of low color and color stable phosphate esters from alkylphenol precursors has been a long recognized problem. In the conventional production of phosphate esters from alkylphenols, undesirable color formation frequently occurs. Color can decrease the phosphate ester's value as a commercial product.
Alkylphenyl esters of phosphoric acid find extensive use as plasticizers for nitrocellulose and polyvinyl chloride (PVC) compositions. In addition, they also serve as additives for gasoline, functional fluids, oils, and are useful as flame retardants in plastics, and the like.
The preparation of alkylphenyl esters of phosphoric acid is generally accomplished by the addition of phosphorus oxychloride, (POCL.sub.3) to selected phenols, such as cresols, xylenols, and the like, and gradually heating the resulting reaction mixture to about 180.degree.C. The reaction is accelerated by the presence of a Friedel-Crafts catalyst such as aluminum chloride, (AlCl.sub.3). In conventional processing, the reaction product is vacuum distilled to remove unreacted phenols as an initial fraction, and the alkylphenyl phosphate ester as a product fraction, leaving high boiling point materials and the catalyst in the residue. The distilled product fraction is then washed thoroughly with sodium hydroxide solution to remove free phenol and acidic materials, followed by water washing. The product fraction is then generally treated with activated carbon to remove color causing impurities.
This process, which works very well with esters produced from conventional by-product alkylphenols, produces unsatisfactory material when applied to mixed alkylphenols produced by the alkylation of phenol with olefins. It is found that the products discolor upon exposure to air, exposure to heat, or storage in the dark. The discoloration has been attributed to the presence of di(o-alkyl) phenols in the alkylated phenol. Steric hindrance caused by the double ortho substitution in 2,6-dialkylphenols and in 2,4,6-trialkylphenols renders the phenols unresponsive to washing with sodium hydroxide solution, so that they are not removed by the caustic wash.
These so-called "hindered phenols," can oxidize in the presence of air to form highly colored quinones, which are the source of undesirable discoloration in the product. These quinones can bleach somewhat in the light, however, color will reappear upon storage in the dark. The color can intensify when the ester is mixed or milled with polyvinyl chloride (PVC) under the influence of air and heat.
Thus, for example, in the case of 2,6-diisopropylphenol, the corresponding diphenoquinone or benzoquinone is highly colored. Esters made from mixtures of alkylated phenols which contain di-orthoalkylphenols such as 2,6-diisopropylphenol can be too highly colored for many uses, particularly for use as plasticizers. High color phosphate esters have limited utility for plasticizer use and are less marketable.
Color formation in phosphate esters is discussed in U.S. Pat. No. 3,681,482, which correlates the degree of color formation in phosphate esters to the degree of alkyl substitution of the aryl ring. Accordingly, tris(methylphenyl) phosphate will produce less color than tris(dimethylphenyl) phosphate. This may be explained by the fact that the tris(methylphenyl) phosphate has only one methyl group substituted on the aryl ring, whereas tris(dimethylphenyl) phosphate has two methyl groups substituted on the aryl ring and, therefore, has more color.
A number of proposals have been made for methods to overcome the problem of undesirable color formation. For example, U.S. Pat. No. 1,958,210 discloses the use of activated carbon to decolorize and remove oxidizable impurities from phosphate esters. This approach is unsatisfactory because activated carbon is not an effective decolorizing agent for alkylphenyl phosphate esters. In certain instances, for example, in the decolorization of isopropylphenyl diphenyl phosphate ester, the use of activated carbon may increase color formation.
U.S. Pat. No. 2,113,951 discloses a method wherein an alkylphenol such as cresylic acid is distilled in the presence of a mineral acid such as sulfuric, hydrochloric or phosphoric acid, to purify it. The purified cresylic acid is then employed in the manufacture of tricresyl phosphate esters which are supposed to be more stable to the action of heat and light than the corresponding ester made from alkylphenols distilled in the absence of an inorganic acid. The disadvantage of this process is that the phenolic residues oxidize to colored quinones, and must be thoroughly distilled in order to remove them and avoid further color formation.
Another method for reducing color is proposed in U.S. Pat. No. 3,681,482 wherein sodium borohydride is used to permanently bleach and color stabilize tris(alkylphenyl)phosphate esters containing 2,6-diisopropylphenol and the corresponding diphenoquinone. The sodium borohydride reduces the diphenoquinone to the colorless 2,6-diisopropylphenol which, however, remains in the product and is a potential source of discoloration if the product is exposed to oxidizing conditions. Sodium borohydride treatment is also expensive in cost of materials and time, as several hours to overnight treating times are necessary.
Thus, it can be seen that the methods proposed in the prior art, are not commercially effective for removing color from alkylphenol esters of phosphoric acid, or do not improve the PVC mill stability when these esters are used as PVC plasticizers.